The present application relates to an optical sheet combination structure improving a luminance by utilizing two optical sheets, a surface emitting device equipped with the optical sheet combination structure, and a liquid crystal display.
A liquid crystal display (LCD) can realize lower consumption power and can be made more compact and thinner than a cathode ray tube (CRT). Liquid crystal displays of various sizes are presently used widely for compact apparatus such as mobile phones, portable game machines, digital cameras, personal digital assistants (PDA's) and for large scale liquid crystal televisions.
Liquid crystal displays are classified into a transmission type, a reflection type and the like. A liquid crystal display, particularly a transmission type liquid crystal display includes a liquid crystal display panel, a first polarizer (polarizing plate) disposed on a light incident side of a liquid crystal display panel and a second polarizer (polarizing plate) disposed on a light emit side of the liquid crystal display panel as well as a backlight unit as an illumination light source. The backlight unit includes a direct-under type disposing a light source directly below a liquid crystal display panel, and in addition an edge light type. A backlight unit of the edge light type is configured by including an light guide plate disposed on the back plane of a liquid crystal display panel, a light source disposed at one edge of the light guide plate, a reflection plate covering the plane opposite to the light emit plane of the light guide plate, and the like.
Cold cathode fluorescent lamps (CCFL) for emitting white light have been used widely as light sources for backlight units of the types described above. Particularly in recent years, a backlight unit using a light emitting diode (LED) as a light source is expected for use with mobile use displays, such as mobile phones or the like. A hot cathode fluorescent lamp (HCFL) can also be used as a light source.
A luminance in a front surface direction is desired for mobile use displays. An approach to restricting a light emit direction of backlight toward the front surface direction has therefore been adopted. For example, there is known an arrangement in which an optical sheet called a luminance improving film or sheet is disposed between a backlight unit and a liquid crystal display panel to direct a light emit direction of backlight toward a front surface direction (refer to Japanese Patent National Publication No. 2002-544565, Patent Document 1, and Japanese Patent Application Publication No. 2004-168869, Patent Document 2).
The luminance improving film is constituted of a prism sheet having triangular prisms arranged on one plane periodically at a micro pitch, and has a function of raising and converging backlight toward the front surface direction. Particularly, there are known an arrangement of superposing two prism sheets by making prism extension directions perpendicular to each other, and an arrangement in which a reflective polarizing sheet is disposed on a prism sheet, the reflective polarizing sheet transmitting ones of linear polarization components and reflecting the others of linear polarization components (refer to Patent Document 1).
FIG. 14 is an exploded perspective view showing an example of the structure of a related art liquid crystal display. The liquid crystal display 1 shown in FIG. 14 includes a liquid crystal display panel 2, a first polarizer 3A and a second polarizer 3B disposed on the light incident and emit sides of the liquid crystal display panel 2, respectively, and a surface light emitter 4. The first and second polarizers 3A and 3B are disposed to make both transmission axes perpendicular to each other (cross Nicol). Although not shown, a phase difference film is often disposed on the liquid crystal display panel 2 to optically compensate for birefringence of a liquid crystal layer and the like.
The surface light emitter 4 is configured by including a light guide plate 5 made of translucent material, a light source 6 disposed on one end of the light guide plate 5, a reflection plate 7 covering the plane opposite to the light output plane of the light guide plate 5. Although the light source 6 is shown as a point light source such as an LED, it may use a linear light source such as a fluorescent lamp.
Sequentially disposed between the light guide plate 5 and first polarizer 3A are a diffusion sheet 8, a first prism sheet 9A, a second prism sheet 9B and a reflective polarizing sheet 10. The first and second prism sheets 9A and 9B are each made of a prism sheet having the same structure, disposed with its prism forming surface being faced toward the liquid crystal display panel 2, and are superposed to make ridge (extension) directions of the prisms perpendicular to each other.
The reflective polarizing sheet 10 has a function of amplifying and transmitting ones of linear polarizing components of light output from the prism sheet 9B and the others of the linear polarizing components has reflecting function. Polarizing components transmitted through the reflective polarizing sheet 10 are input to the liquid crystal display panel 2 through the transmission axis of the first polarizer 3A. It is possible to increase a front surface luminance at each layer, by disposing the reflective polarizing sheet 10 so as to make the linear polarizing components in an amplification direction parallel to the transmission axis direction of the first polarizer 3A.
The prism sheets 9A and 9B are generally formed by laminating a curing resin layer made of active energy ray curing resin on the surface of a transparent base (refer to Patent Document 2). FIG. 15 is a schematic diagram showing the structure of the related art prism sheets 9A and 9B hereinafter collectively called a prism sheet 9, unless the sheets are described separately). The prism sheet 9 has a structure that a curing resin layer 12 is laminated integrally on the surface of a transparent base 11. The curing resin layer 12 is formed of active energy ray curing resin which is cured upon exposure to an active energy ray such as an ultraviolet ray and an electron beam.
The curing resin layer 12 of the related art prism sheet 9 is constituted of a number of triangular prisms 12a disposed at a constant pitch and a skirt layer (base layer) 12c for supporting a number of prisms 12a. A total thickness (H) of the prism sheet 9 is represented by a sum of a thickness (D2) of the transparent base 11 and a thickness (D3) of the curing resin layer 12. A height of the prism 12a is represented by a depth (D1) of a valley portion 12b of adjacent prisms 12a. The skirt layer 12c is interposed between the surface of the transparent base 11 and the prisms 12a, and its thickness (ΔDy) corresponds to a difference (D3-D1) between the thickness of the curing resin layer 12 and the height of the prism 12a. 
In recent mobile use displays, thinning of the entire display and realizing high image quality has been required In the related art liquid crystal display described above, at least three or more optical sheets including the first and second prism sheets 9A and 9B and reflective polarizing sheet 10 are required in order to improve a front surface luminance of the liquid crystal display panel. Since there is a limit in thinning and thickening each optical sheet, it is very difficult to thin considerably a total thickness of optical sheets (a total thickness of the three-sheet configuration described above is about 250 μm). For example, the reflective polarizing sheet 10 shown in FIG. 14 is particularly difficult to thin, because this sheet is a laminated type sheet. Therefore, the structure of the related art liquid crystal display 1 results in lower productivity and increased manufacture cost and in difficulty of thinning the whole display, because of an increase in the number of used sheets.
Particularly in a mobile use liquid crystal display, the transmission axis of the first polarizer 3A on the backlight side is often disposed by rotating several tens of degrees at maximum from the cross Nicol state relative to the second polarizer 3B, in order to optimize the display characteristics such as contrast when a phase difference film is combined. In this case, an angle shift between the transmission axis of the first polarizer 3A and the polarizing components outputted from the reflective polarizing sheet 10 becomes large, thereby posing a problem that the front surface luminance is lowered greatly. FIG. 16 shows an example of a relation between a rotation angle of the transmission axis of the first polarizer with respect to the transmission axis of the second polarizer and a front surface luminance, with the front surface luminance being set to “1” in the cross Nicol state. As apparent from FIG. 16, a reduction rate of a luminance is large relative to a rotation angle of the transmission axis of the first polarizer. In the related art liquid crystal display, as the rotation adjustment is performed for the transmission axis of the first polarizer 3A, an optical axis of the reflective polarizing sheet 10 is also required to be adjusted, arising a possibility of lowering assembly efficiency or productivity.